Hybrid vehicle

ABSTRACT

A hybrid vehicle includes a clutch and a transmission unit configured to be able to switch into a neutral state. The hybrid vehicle is able to transmit power of an engine through any one of a first path through which power is transmitted from the engine to a first MG via a transmission unit and a differential unit and another second path through which power is transmitted from the engine to the first MG. The clutch is provided in the second path, and switches between an engaged state and a released state. The controller controls the engine, the first MG, the transmission unit and the clutch. The controller sets the transmission unit to a non-neutral state, sets the clutch to the engaged state, and then causes the vehicle to travel by using driving force from the first MG and driving force from the second MG.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a hybrid vehicle and, more particularly, to ahybrid vehicle including first and second rotary electric machines and atransmission unit.

2. Description of Related Art

There is known a hybrid vehicle including not only an engine, two rotaryelectric machines and a power split mechanism but also a transmissionmechanism between the engine and the power split mechanism.

A hybrid vehicle described in International Application Publication No.2013/114594 employs a series-parallel hybrid system. In the vehiclehaving a series-parallel hybrid system, the power of an engine istransmitted to a first rotary electric machine (first motor generator)and is used to generate electric power, while part of the power of theengine is also transmitted to drive wheels via a power split mechanism.

The vehicle described in International Application Publication No.2013/114594 is allowed to travel by setting both a first rotary electricmachine and a second rotary electric machine in a motoring state whilestopping an engine. In this case, the torque of each of the two motorgenerators is used as torque for rotating drive wheels. However, withthe configuration of the vehicle described in the InternationalApplication Publication No. 2013/114594, such a driving method isallowed only when the rotation speed of the engine is zero. When theengine is rotated, the vehicle is operated in the above-describedseries-parallel hybrid mode, so the first rotary electric machine isused to generate electric power. Because the first rotary electricmachine is in a regeneration state, torque during motoring of the firstrotary electric machine cannot be directly used to rotate the drivewheels.

Even when the rotation speed of the engine is not zero, but when thetorque of the first rotary electric machine is allowed to be directlyused to rotate the drive wheels, it is desirable because it is possibleto increase driving torque of the vehicle.

SUMMARY OF THE INVENTION

The invention provides a hybrid vehicle that has an increased number ofopportunities that the torque of each of two motor generators is used todrive a wheel.

An aspect of the invention provides a hybrid vehicle. The hybrid vehicleincludes an internal combustion engine, a first rotary electric machine,a second rotary electric machine, a power transmission unit, adifferential unit, a clutch and a controller.

The second rotary electric machine is configured to output power to adrive wheel. The power transmission unit includes an input element andan output element. The input element is configured to receive power fromthe internal combustion engine. The output element is configured tooutput power input to the input element. The power transmission unit isconfigured to switch between a non-neutral state where power istransmitted between the input element and the output element and aneutral state where power is not transmitted between the input elementand the output element.

The differential unit includes a first rotating element, a secondrotating element and a third rotating element. The first rotatingelement is connected to the first rotary electric machine. The secondrotating element is connected to the second rotary electric machine andthe drive wheel. The third rotating element is connected to the outputelement of the power transmission unit. The differential unit isconfigured such that, when rotation speeds of any two of the firstrotating element, the second rotating element and the third rotatingelement are determined, a rotation speed of the remaining one of thefirst rotating element, the second rotating element and the thirdrotating element is determined.

The clutch is configured to switch between an engaged state where poweris transmitted from the internal combustion engine to the first rotaryelectric machine and a released state where transmission of power fromthe internal combustion engine to the first rotary electric machine isinterrupted. Power from the internal combustion engine is transmitted tothe first rotary electric machine though at least one of a first path ora second path. The first path is a path through which power istransmitted from the internal combustion engine to the first rotaryelectric machine via the power transmission unit and the differentialunit, and the second path is a path through which power is transmittedfrom the internal combustion engine to the first rotary electric machinevia a path different from the first path. The clutch is provided in thesecond path.

The controller is configured to (i) control the internal combustionengine, the first rotary electric machine, the power transmission unitand the clutch, and (ii) set the power transmission unit to thenon-neutral state, set the clutch to the engaged state, and then causethe vehicle to travel by using driving force from the first rotaryelectric machine and driving force from the second rotary electricmachine.

With the above-described hybrid vehicle, the vehicle is configured asdescribed above, and the power transmission unit, the clutch, the firstrotary electric machine and the second rotary electric machine arecontrolled as described above. Thus, the vehicle is allowed to bepropelled by operating both the first rotary electric machine and thesecond rotary electric machine to carry out motoring even in a statewhere the rotation speed of the internal combustion engine is not zero.Therefore, it is possible to increase the opportunity that the torque ofthe two rotary electric machines is used as driving torque of thevehicle, so the flexibility of control over the vehicle increases in thecase where large driving force is required during traveling.

In the hybrid vehicle, the controller may be configured to switch adrive mode of the vehicle between a first mode and a second mode inresponse to a vehicle speed. The first mode is a drive mode in which arotation speed of the internal combustion engine is fixed to zero, theclutch is set to the released state and then the vehicle is caused totravel by using driving force from the first rotary electric machine anddriving force from the second rotary electric machine. The second modeis a drive mode in which the power transmission unit is set to thenon-neutral state, the clutch is set to the engaged state and then thevehicle is caused to travel by using driving force from the first rotaryelectric machine and driving force from the second rotary electricmachine.

With the above-described hybrid vehicle, because the second mode isprovided as the drive mode, even when the rotation speed of the engineis not zero like a transition from a state where the engine is operatedto an EV mode, the vehicle is allowed to travel with large driving forceusing driving force from the first rotary electric machine and drivingforce from the second rotary electric machine.

In the hybrid vehicle, the controller may be configured to (i) when thevehicle speed is lower than a determination threshold, set the drivemode to the first mode, and (ii) when the vehicle speed is higher thanthe determination threshold, set the drive mode to the second mode.

With the above-described hybrid vehicle, when the drive mode is selectedas described above, even when the vehicle speed increases and thevehicle is not allowed to travel in first mode because of the limitationof the rotation speed of the first rotary electric machine, the vehicleis allowed to travel with large driving force using driving force fromthe first rotary electric machine and driving force from the secondrotary electric machine when the second mode is used.

In the hybrid vehicle, the controller may be configured to (i) in thirdmode that is the drive mode of the vehicle, set the power transmissionunit to the non-neutral state, set the clutch to the released state, andthen cause the first rotary electric machine to generate electric powerin a state where the internal combustion engine is operated, and causethe second rotary electric machine to generate driving force forpropelling the vehicle, and (ii) when the drive mode is changed from thethird mode to the first mode, change the drive mode via the second mode.

With the above-described hybrid vehicle, when the drive mode is changedfrom the third mode to the first mode, it is possible not to cause adriver to experience a feeling of output torque loss by changing thedrive mode via the second mode.

In the hybrid vehicle, the controller may be configured to, when fuel isnot supplied to the internal combustion engine in the case where thevehicle is caused to travel in second mode, change open or close timingof at least one of an intake valve or exhaust valve such that resistanceis reduced during rotation of the internal combustion engine.

When the vehicle is caused to travel in second mode and fuel is notsupplied to the internal combustion engine, the internal combustionengine is forcibly rotated by the first rotary electric machine and thesecond rotary electric machine. In this case, an energy loss is smallerwhen the rotation resistance of the internal combustion engine is small.In order to reduce the rotation resistance of the internal combustionengine, it is desirable that the compressibility and expansioncoefficient of air in a cylinder be small. Therefore, with theabove-described hybrid vehicle, the controller reduces the rotationresistance of the internal combustion engine by changing the open/closetiming of the intake valve or exhaust valve, thus reducing an energyloss.

In the hybrid vehicle, the controller may be configured to set the powertransmission unit to the non-neutral state, set the clutch to theengaged state, and then cause the vehicle to travel by using drivingforce from the internal combustion engine in addition to driving forcefrom the first rotary electric machine and driving force from the secondrotary electric machine.

With the above-described hybrid vehicle, through the above-describedcontrol, it is possible to further increase the maximum driving force ofthe vehicle as compared to the EV mode in which the internal combustionengine is stopped and the first rotary electric machine and the secondrotary electric machine are operated to carry out motoring.

In the hybrid vehicle, the controller may be configured to, in fourthmode as the drive mode of the vehicle, set the power transmission unitto the non-neutral state, set the clutch to the engaged state, and thencause the vehicle to travel by using driving force from the internalcombustion engine in a state where the first rotary electric machine andthe second rotary electric machine are not caused to generate torque.

With the above-described hybrid vehicle, through the above-describedcontrol, in an operating range in which the internal combustion engineis efficiently operable, the power of the internal combustion engine isallowed to be directly transmitted to the drive wheel without beingconverted to electric power, so it is possible to improve fuel economy.

In the hybrid vehicle, the controller may be configured to, in fifthmode as the drive mode of the vehicle, set the power transmission unitto the neutral state, set the clutch to the engaged state, and thencause the first rotary electric machine to generate electric power byusing power of the internal combustion engine, and cause the secondrotary electric machine to generate driving force for propelling thevehicle.

With the above-described hybrid vehicle, in the above-described seriesHV mode, a shock at a startup of the internal combustion engine isinterrupted by the power transmission unit in the neutral state, and isnot transmitted to the drive wheel. Thus, it is possible to reduce ashock at a startup of the internal combustion engine, which isexperienced by a user.

In the hybrid vehicle, the power transmission unit may be configured tobe able to change the ratio of a rotation speed of the input element toa rotation speed of the output element.

With the above-described hybrid vehicle, it is possible to increase thestate of the vehicle where it is possible to set the EV mode in whichlarge driving force is generated by operating both the two rotaryelectric machines to carry out motoring.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a view that shows the overall configuration of a hybridvehicle including a drive system according to an embodiment of theinvention;

FIG. 2 is a block diagram that schematically shows power transmissionpaths of components of the vehicle in FIG. 1;

FIG. 3 is a block diagram that shows the configuration of a controllerfor the vehicle in FIG. 1;

FIG. 4 is a view that schematically shows the configuration of ahydraulic circuit mounted on the hybrid vehicle shown in FIG. 1;

FIG. 5 is a chart that shows each drive mode in the hybrid vehicle andcontrolled statuses of clutches and brake of a transmission unit in eachdrive mode;

FIG. 6 is a nomograph for illustrating an operation of a one-motor EVmode (E1 line in FIG. 5) in the hybrid vehicle;

FIG. 7 is a nomograph for illustrating an operation of a two-motor EVmode (E3 line in FIG. 5) in the hybrid vehicle;

FIG. 8 is a nomograph for illustrating an operation of a(series-parallel) HV mode (H1, H2 lines in FIG. 5) in the hybridvehicle;

FIG. 9 is a nomograph for illustrating an operation of a (series) HVmode (H4 line in FIG. 5) in the hybrid vehicle;

FIG. 10 is a nomograph for illustrating an operation of a two-motor EVmode (E4, E5 lines in FIG. 5) in the hybrid vehicle;

FIG. 11 is a nomograph for illustrating an operation of a (parallel) HVmode (H7, H9 lines in FIG. 5) in the hybrid vehicle;

FIG. 12 is a nomograph for illustrating an operation of an engine drivemode (Z1 line in FIG. 5) in the hybrid vehicle;

FIG. 13 is a nomograph for illustrating an operation of an engine drivemode (Z2 line in FIG. 5) in the hybrid vehicle;

FIG. 14 is a graph that shows the relationship between a vehicle speedand a maximum driving force in each drive mode in the hybrid vehicle;

FIG. 15 is a flowchart for illustrating control over the clutches andthe brake in two-motor EV mode in the hybrid vehicle;

FIG. 16 is a view that shows an example of a map for determining thedrive mode in the hybrid vehicle; and

FIG. 17 is an operation waveform chart that shows an example of a changefrom the (series-parallel) HV mode to the two-motor EV mode in thehybrid vehicle.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the invention will be described withreference to the accompanying drawings. Like reference numerals denotethe same or corresponding portions in the following embodiment, and thedescription thereof will not be repeated.

FIG. 1 is a view that shows the overall configuration of a hybridvehicle including a drive system according to the embodiment of theinvention.

As shown in FIG. 1, the hybrid vehicle 1 (hereinafter, also referred toas vehicle 1) includes an engine 10, the drive system 2, drive wheels 90and a controller 100. The drive system 2 includes a first motorgenerator (hereinafter, referred to as first MG) 20 that is a firstrotary electric machine, a second motor generator (hereinafter, referredto as second MG) 30 that is a second rotary electric machine, atransmission unit 40, a differential unit 50, a clutch CS, an inputshaft 21, a counter shaft 70 that is an output shaft of the drive system2, a differential gear set 80 and a hydraulic circuit 500.

The hybrid vehicle 1 is a front-engine front-drive (FF) hybrid vehiclethat travels by using the power of at least any one of the engine 10,the first MG 20 or the second MG 30. The hybrid vehicle 1 may be aplug-in hybrid vehicle of which an in-vehicle battery (not shown) isrechargeable from an external power supply.

The engine 10 is, for example, an internal combustion engine, such as agasoline engine and a diesel engine. Each of the first MG 20 and thesecond MG 30 is, for example, a permanent magnet synchronous motor thatincludes a rotor in which permanent magnets are embedded. The drivesystem 2 is a double-axis drive system in which the first MG 20 isprovided along a first axis 12 coaxial with the crankshaft of the engine10 and the second MG 30 is provided along a second axis 14 differentfrom the first axis 12. The first axis 12 and the second axis 14 areparallel to each other.

The transmission unit 40, the differential unit 50 and the clutch CS arefurther provided along the first axis 12. The transmission unit 40, thedifferential unit 50, the first MG 20 and the clutch CS are arrangedfrom the side close to the engine 10 in the stated order.

The first MG 20 is provided so as to be able to receive power from theengine 10. More specifically, the input shaft 21 of the drive system 2is connected to the crankshaft of the engine 10. The input shaft 21extends along the first axis 12 in a direction away from the engine 10.The input shaft 21 is connected to the clutch CS at its distal endextending from the engine 10. A rotary shaft 22 of the first MG 20extends in a cylindrical shape along the first axis 12. The input shaft21 passes through the inside of the rotary shaft 22 at a portion beforethe input shaft 21 is connected to the clutch CS. The input shaft 21 isconnected to the rotary shaft 22 of the first MG 20 via the clutch CS.

The clutch CS is provided in a power transmission path from the engine10 to the first MG 20. The clutch CS is a hydraulic friction engagementelement that is able to couple the input shaft 21 to the rotary shaft 22of the first MG 20. When the clutch CS is placed in an engaged state;the input shaft 21 and the rotary shaft 22 are coupled to each other,and transmission of power from the engine 10 to the first MG 20 isallowed. When the clutch CS is placed in a released state, coupling ofthe input shaft 21 to the rotary shaft 22 is released, and transmissionof power from the engine 10 to the first MG 20 via the clutch CS isinterrupted.

The transmission unit 40 shifts power from the engine 10 and thenoutputs the power to the differential unit 50. The transmission unit 40includes a single-pinion-type planetary gear mechanism, a clutch C1 anda brake B1. The single-pinion-type planetary gear mechanism includes asun gear S1, pinions P1, a ring gear R1 and a carrier CA1.

The sun gear S1 is provided such that the rotation center of the sungear S1 coincides with the first axis 12. The ring gear R1 is providedcoaxially with the sun gear S1 on the radially outer side of the sungear S1. The pinions P1 are arranged between the sun gear S1 and thering gear R1, and are in mesh with the sun gear S1 and the ring gear R1.The pinions P1 are rotatably supported by the carrier CA1. The carrierCA1 is connected to the input shaft 21, and rotates integrally with theinput shaft 21. Each of the pinions P1 is provided so as to berevolvable about the first axis 12 and rotatable around the central axisof the pinion P1.

As shown in FIG. 6 to FIG. 13 (described later), the rotation speed ofthe sun gear S1, the rotation speed of the carrier CA1 (that is, therotation speed of the engine 10) and the rotation speed of the ring gearR1 are in the relationship represented by points that are connected by astraight line in each of the nomographs (that is, the relationship that,when any two rotation speeds are determined, the remaining one rotationspeed is also determined).

In the present embodiment, the carrier CA1 is provided as an inputelement to which power is input from the engine 10, and the ring gear R1is provided as an output element that outputs the power input to thecarrier CA1. By the use of the planetary gear mechanism including thesun gear S1, the pinions P1, the ring gear R1 and the carrier CA1, powerinput to the carrier CA1 is shifted and output from the ring gear R1.

The clutch C1 is a hydraulic friction engagement element that is able tocouple the sun gear S1 to the carrier CA1. When the clutch C1 is placedin an engaged state, the sun gear S1 and the carrier CA1 are coupled toeach other, and rotate integrally with each other. When the clutch C1 isplaced in a released state, integral rotation of the sun gear S1 and thecarrier CA1 is cancelled.

The brake B1 is a hydraulic friction engagement element that is able torestrict (lock) the rotation of the sun gear S1. When the brake B1 isplaced in an engaged state, the sun gear S1 is fixed to the case body ofthe drive system, and the rotation of the sun gear S1 is restricted.When the brake B1 is placed in a released state (disengaged state), thesun gear S1 is separated from the case body of the drive system, and therotation of the sun gear S1 is allowed.

A separating wall W1 is provided between the planetary gear mechanismand the brake B1. It is preferable to provide the separating wall W1 atthis position, because it is possible to provide an oil passage in theseparating wall W1 in order to supply activating oil of the brake B1 andthe clutch C1. Furthermore, a hole which is provided in the separatingwall W1 can be small by providing the separating wall W1, the brake B1,the clutch C1 and the engine 10 in this described order and by makingthe carrier CA1 serve as an inner rotating element and the sun gear S1serve as an outer rotating element.

A speed ratio (the ratio of the rotation speed of the carrier CA1 thatis the input element to the rotation speed of the ring gear R1 that isthe output element, specifically, Rotation Speed of Carrier CA1/RotationSpeed of Ring Gear R1) of the transmission unit 40 is changed inresponse to a combination of the engaged/released states of the clutchC1 and brake B1. When the clutch C1 is engaged and the brake B1 isreleased, a low gear position Lo in which the speed ratio is 1.0(directly coupled state) is established. When the clutch C1 is releasedand the brake B1 is engaged, a high gear position Hi in which the speedratio is smaller than 1.0 (for example, 0.7, and a so-called over-drivestate) is established. When the clutch C1 is engaged and the brake B1 isengaged, the rotation of the sun gear S1 and the rotation of the carrierCA1 are restricted, so the rotation of the ring gear R1 is alsorestricted.

The transmission unit 40 is configured to be able to switch between anon-neutral state and a neutral state. In the non-neutral state, poweris transmitted. In the neutral state, power is not transmitted. In thepresent embodiment, the above-described directly coupled state andover-drive state correspond to the non-neutral state. On the other hand,when both the clutch C1 and the brake B1 are released, the carrier CA1is allowed to coast about the first axis 12. Thus, the neutral state inwhich power transmitted from the engine 10 to the carrier CA1 is nottransmitted from the carrier CA1 to the ring gear R1 is obtained.

The differential unit 50 includes a single-pinion-type planetary gearmechanism and a counter drive gear 51. The single-pinion-type planetarygear mechanism includes a sun gear S2, pinions P2, a ring gear R2 and acarrier CA2.

The sun gear S2 is provided such that the rotation center of the sungear S2 coincides with the first axis 12. The ring gear R2 is providedcoaxially with the sun gear S2 on the radially outer side of the sungear S2. The pinions P2 are arranged between the sun gear S2 and thering gear R2, and are in mesh with the sun gear S2 and the ring gear R2.The pinions P2 are rotatably supported by the carrier CA2. The carrierCA2 is connected to the ring gear R1 of the transmission unit 40, androtates integrally with the ring gear R1. Each of the pinions P2 isprovided so as to be revolvable about the first axis 12 and rotatablearound the central axis of the pinion P2.

The rotary shaft 22 of the first MG 20 is connected to the sun gear S2.The rotary shaft 22 of the first MG 20 rotates integrally with the sungear S2. The counter drive gear 51 is connected to the ring gear R2. Thecounter drive gear 51 is an output gear of the differential unit 50. Theoutput gear rotates integrally with the ring gear R2.

As shown in FIG. 6 to FIG. 13 (described later), the rotation speed ofthe sun gear S2 (that is, the rotation speed of the first MG 20), therotation speed of the carrier CA2 and the rotation speed of the ringgear R2 are in the relationship represented by points that are connectedby a straight line in each of the nomographs (that is, the relationshipthat, when any two rotation speeds are determined, the remaining onerotation speed is also determined). Therefore, when the rotation speedof the carrier CA2 is a predetermined value, it is possible tosteplessly change the rotation speed of the ring gear R2 by adjustingthe rotation speed of the first MG 20.

The counter shaft 70 extends parallel to the first axis 12 and thesecond axis 14. The counter shaft 70 is arranged parallel to the rotaryshaft 22 of the first MG 20 and a rotary shaft 31 of the second MG 30. Adriven gear 71 and a drive gear 72 are provided on the counter shaft 70.The driven gear 71 is in mesh with the counter drive gear 51 of thedifferential unit 50. That is, the power of the engine 10 and the powerof the first MG 20 are transmitted to the counter shaft 70 via thecounter drive gear 51 of the differential unit 50.

The transmission unit 40 and the differential unit 50 are connected inseries with each other in a power transmission path from the engine 10to the counter shaft 70. Therefore, power from the engine 10 is shiftedin the transmission unit 40 and the differential unit 50 and thentransmitted to the counter shaft 70.

The driven gear 71 is in mesh with a reduction gear 32 connected to therotary shaft 31 of the second MG 30. That is, the power of the second MG30 is transmitted to the counter shaft 70 via the reduction gear 32.

The drive gear 72 is in mesh with a differential ring gear 81 of thedifferential gear set 80. The differential gear set 80 is connected tothe right and left drive wheels 90 via corresponding right and leftdrive shafts 82. That is, the rotation of the counter shaft 70 istransmitted to the right and left drive shafts 82 via the differentialgear set 80.

With the above-described configuration in which the clutch CS isprovided, the hybrid vehicle 1 is allowed to operate in a mode in whicha series-parallel system is used (hereinafter, referred to asseries-parallel mode) and is also allowed to operate in a mode in whicha series system is used (hereinafter, referred to as series mode). Interms of this point, how power is transmitted from the engine in eachmode will be described with reference to the schematic view shown inFIG. 2.

FIG. 2 is a block diagram that schematically shows power transmissionpaths of components of the vehicle in FIG. 1. As shown in FIG. 2, thehybrid vehicle 1 includes the engine 10, the first MG 20, the second MG30, the transmission unit 40, the differential unit 50, a battery 60 andthe clutch CS.

The second MG 30 is provided so as to be able to output power to thedrive wheels 90. The transmission unit 40 includes the input element andthe output element. The power of the engine 10 is input to the inputelement. The output element outputs the power input to the inputelement. The transmission unit 40 is configured to be able to switchbetween the non-neutral state and the neutral state. In the non-neutralstate, power is transmitted between the input element and the outputelement. In the neutral state, power is not transmitted between theinput element and the output element.

The battery 60 supplies electric power to the first MG 20 or the secondMG 30 during motoring of a corresponding one of the first MG 20 and thesecond MG 30, and stores electric power generated by the first MG 20 orthe second MG 30 during regeneration of a corresponding one of the firstMG 20 and the second MG 30.

The differential unit 50 includes a first rotating element, a secondrotating element and a third rotating element. The first rotatingelement is connected to the first MG 20. The second rotating element isconnected to the second MG 30 and the drive wheels 90. The thirdrotating element is connected to the output element of the transmissionunit 40. The differential unit 50 is configured as in the case of, forexample, the planetary gear mechanism, or the like, such that, when therotation speeds of any two of the first to third rotating elements aredetermined, the rotation speed of the remaining one of the first tothird rotating elements is determined.

The hybrid vehicle 1 is configured to be able to transmit power from theengine 10 to the first MG 20 with the use of at least any one of twopaths K1, K2 through which power is transmitted. The path K1 is a paththrough which power is transmitted from the engine 10 to the first MG 20via the transmission unit 40 and the differential unit 50. The path K2is different from the path K1, and is a path through which power istransmitted from the engine 10 to the first MG 20. The clutch CS isprovided in the path K2, and is able to switch between the engaged stateand the released state. In the engaged state, power is transmitted fromthe engine 10 to the first MG 20. In the released state, transmission ofpower from the engine 10 to the first MG 20 is interrupted.

In HV mode in which the engine is operated, any one of the clutch C1 andthe brake B1 is placed in the engaged state, and the other one of theclutch C1 and the brake B1 is placed in the released state. Thus, whenthe transmission unit 40 is controlled to the non-neutral state, poweris transmitted from the engine 10 to the first MG 20 through the pathK1. At this time, when the clutch CS is placed in the released state tointerrupt the path K2 at the same time, the vehicle is operable inseries-parallel mode.

On the other hand, in HV mode in which the engine is operated, whenpower is transmitted through the path K2 by directly coupling the engine10 to the first MG 20 with the clutch CS and the path K1 is interruptedby controlling the transmission unit 40 such that the transmission unit40 is placed in the neutral state by placing both the clutch C1 and thebrake B1 in the released state, the vehicle is operable in series mode.At this time, in the differential unit 50, the rotating elementconnected to the transmission unit 40 is freely rotatable, so the othertwo rotating elements do not influence each other and are rotatable.Therefore, it is possible to independently perform the operation ofgenerating electric power by rotating the first MG 20 with the use ofthe rotation of the engine 10 and the operation of rotating the drivewheels by driving the second MG 30 with the use of generated electricpower or electric power charged in the battery 60.

When the engine 10 and the first MG 20 are directly coupled to eachother by the clutch CS and the transmission unit 40 is controlled to thenon-neutral state in a state where the engine 10 is operated, therotation of the engine 10 is transmitted to the drive wheels at a fixedgear ratio. At this time, power is not transmitted via a route, such asthe paths K1, K2, but power is transmitted from the engine 10 to thedrive wheels 90 via the differential unit 50.

The transmission unit 40 does not always need to be able to change thespeed ratio. As long as it is possible to interrupt transmission ofpower between the engine 10 and the differential unit 50 in the path K1,a mere clutch is applicable.

FIG. 3 is a block diagram that shows the configuration of the controller100 of the vehicle shown in FIG. 1. As shown in FIG. 3, the controller100 includes an HV ECU 150, an MG ECU 160 and an engine ECU 170. Each ofthe HV ECU 150, the MG ECU 160 and the engine ECU 170 is an electroniccontrol unit including a computer. The number of ECUs is not limited tothree. An integrated single ECU may be provided as a whole, or two orfour or more of split ECUs may be provided.

The MG ECU 160 controls the first MG 20 and the second MG 30. The MG ECU160, for example, controls the output torque of the first MG 20 byadjusting the value of current that is supplied to the first MG 20, andcontrols the output torque of the second MG 30 by adjusting the value ofcurrent that is supplied to the second MG 30.

The engine ECU 170 controls the engine 10. The engine ECU 170, forexample, controls the opening degree of an electronic throttle valve ofthe engine 10, controls ignition of the engine by outputting an ignitionsignal, or controls injection of fuel to the engine 10. The engine ECU170 controls the output torque of the engine 10 through opening degreecontrol over the electronic throttle valve, injection control, ignitioncontrol, and the like.

The HV ECU 150 comprehensively controls the entire vehicle. A vehiclespeed sensor, an accelerator operation amount sensor, an MG1 rotationspeed sensor, an MG2 rotation speed sensor, an output shaft rotationspeed sensor, a battery sensor, and the like, are connected to the HVECU 150. With these sensors, the HV ECU 150 acquires a vehicle speed, anaccelerator operation amount, the rotation speed of the first MG 20, therotation speed of the second MG 30, the rotation speed of the outputshaft of a power transmission system, a battery state SOC, and the like.

The HV ECU 150 calculates a required driving force, a required power, arequired torque, and the like, for the vehicle on the basis of acquiredinformation. The HV ECU 150 determines the output torque of the first MG20 (hereinafter, also referred to as MG1 torque), the output torque ofthe second MG 30 (hereinafter, also referred to as MG2 torque) and theoutput torque of the engine 10 (hereinafter, also referred to as enginetorque) on the basis of the calculated required values. The HV ECU 150outputs a command value of the MG1 torque and a command value of the MG2torque to the MG ECU 160. The HV ECU 150 outputs a command value of theengine torque to the engine ECU 170.

The HV ECU 150 controls the clutches C1, CS and the brake B1 on thebasis of the drive mode (described later), and the like. The HV ECU 150outputs, to the hydraulic circuit 500 shown in FIG. 1, a command value(PbC1) of hydraulic pressure that is supplied to the clutch C1, acommand value (PbCS) of hydraulic pressure that is supplied to theclutch CS and a command value (PbB1) of hydraulic pressure that issupplied to the brake B1. The HV ECU 150 outputs a control signal NM anda control signal S/C to the hydraulic circuit 500 shown in FIG. 1.

The hydraulic circuit 500 shown in FIG. 1 controls hydraulic pressuresthat are respectively supplied to the clutch C1 and the brake B1 inresponse to the command values PbC1, PbB1, controls an electric oil pumpin response to the control signal NM, and controls whether to allow orprohibit simultaneous engagement of the clutch C1, the brake B1 and theclutch CS in response to the control signal S/C.

Next, the configuration of the hydraulic circuit will be described. FIG.4 is a view that schematically shows the configuration of the hydrauliccircuit 500 mounted on the hybrid vehicle 1. The hydraulic circuit 500includes a mechanical oil pump (hereinafter, also referred to as MOP)501, the electric oil pump (hereinafter, also referred to as EOP) 502,pressure regulating valves 510, 520, linear solenoid valves SL1, SL2,SL3, simultaneous supply prevention valves 530, 540, 550, anelectromagnetic change-over valve 560, a check valve 570, and oilpassages LM, LE, L1, L2, L3, L4.

The MOP 501 is driven by the rotation of the carrier CA2 of thedifferential unit 50 to generate hydraulic pressure. Therefore, when thecarrier CA2 is rotated by, for example, driving the engine 10, the MOP501 is also driven; whereas, when the carrier CA2 is stopped, the MOP501 is also stopped. The MOP 501 outputs generated hydraulic pressure tothe oil passage LM.

The hydraulic pressure in the oil passage LM is regulated (reduced) to apredetermined pressure by the pressure regulating valve 510.Hereinafter, the hydraulic pressure in the oil passage LM, regulated bythe pressure regulating valve 510, is also referred to as line pressurePL. The line pressure PL is supplied to each of the linear solenoidvalves SL1, SL2, SL3.

The linear solenoid valve SL1 generates hydraulic pressure for engagingthe clutch C1 (hereinafter, referred to as C1 pressure) by regulatingthe line pressure PL in response to the hydraulic pressure command valuePbC1 from the controller 100. The C1 pressure is supplied to the clutchC1 via the oil passage L1.

The linear solenoid valve SL2 generates hydraulic pressure for engagingthe brake B1 (hereinafter, referred to as B1 pressure) by regulating theline pressure PL in response to the hydraulic pressure command valuePbB1 from the controller 100. The B1 pressure is supplied to the brakeB1 via the oil passage L2.

The linear solenoid valve SL3 generates hydraulic pressure for engagingthe clutch CS (hereinafter, referred to as CS pressure) by regulatingthe line pressure PL in response to the hydraulic pressure command valuePbCS from the controller 100. The CS pressure is supplied to the clutchCS via the oil passage L3.

The simultaneous supply prevention valve 530 is provided in the oilpassage L1, and is configured to prevent the clutch C1 and at least oneof the brake B1 or the clutch CS from being simultaneously engaged.Specifically, the oil passages L2, L3 are connected to the simultaneoussupply prevention valve 530. The simultaneous supply prevention valve530 operates by using the B1 pressure and the CS pressure through theoil passages L2, L3 as signal pressures.

When both signal pressures that are the B1 pressure and the CS pressureare not input to the simultaneous supply prevention valve 530 (that is,when both the brake B1 and the clutch CS are released), the simultaneoussupply prevention valve 530 is in a normal state in which the C1pressure is supplied to the clutch C1. FIG. 4 illustrates the case wherethe simultaneous supply prevention valve 530 is in the normal state.

On the other hand, when at least one of the signal pressures that arethe B1 pressure and the CS pressure is input to the simultaneous supplyprevention valve 530 (that is, when at least one of the brake B1 or theclutch CS is engaged), even when the clutch C1 is engaged, thesimultaneous supply prevention valve 530 switches into a drain state inwhich supply of the C1 pressure to the clutch C1 is cut off and thehydraulic pressure in the clutch C1 is released to the outside. Thus,the clutch C1 is released, so the clutch C1 and at least one of thebrake B1 or the clutch CS are prevented from being simultaneouslyengaged.

Similarly, the simultaneous supply prevention valve 540 operates inresponse to the C1 pressure and the CS pressure as signal pressures toprevent the brake B1 and at least one of the clutch C1 or the clutch CSfrom being simultaneously engaged. Specifically, when both the signalpressures that are the C1 pressure and the CS pressure are not input tothe simultaneous supply prevention valve 540, the simultaneous supplyprevention valve 540 is in a normal state in which the B1 pressure issupplied to the brake B1. On the other hand, when at least one of thesignal pressures that are the C1 pressure and the CS pressure is inputto the simultaneous supply prevention valve 540, the simultaneous supplyprevention valve 540 switches into a drain state in which supply of theB1 pressure to the brake B1 is cut off and the hydraulic pressure in thebrake B1 is released to the outside. FIG. 4 illustrates the case wherethe C1 pressure is input to the simultaneous supply prevention valve 540as the signal pressure and the simultaneous supply prevention valve 540is in the drain state.

Similarly, the simultaneous supply prevention valve 550 operates byusing the C1 pressure and the B1 pressure as signal pressures to preventthe clutch CS and at least one of the clutch C1 or the brake B1 frombeing simultaneously engaged. Specifically, when both the signalpressures that are the C1 pressure and the B1 pressure are not input tothe simultaneous supply prevention valve 550, the simultaneous supplyprevention valve 550 is in a normal state in which the CS pressure issupplied to the clutch CS. On the other hand, when at least one of thesignal pressures that are the C1 pressure and the B1 pressure is inputto the simultaneous supply prevention valve 550, the simultaneous supplyprevention valve 550 switches into a drain state in which supply of theCS pressure to the clutch CS is cut off and the hydraulic pressure inthe clutch CS is released to the outside. FIG. 4 illustrates the casewhere the C1 pressure is input to the simultaneous supply preventionvalve 550 and the simultaneous supply prevention valve 550 is in thedrain state.

The EOP 502 is driven by a motor (hereinafter, also referred to asinternal motor) 502A provided inside to generate hydraulic pressure. Theinternal motor 502A is controlled by the control signal NM from thecontroller 100. Therefore, the EOP 502 is operable irrespective ofwhether the carrier CA2 is rotating. The EOP 502 outputs generatedhydraulic pressure to the oil passage LE.

The hydraulic pressure in the oil passage LE is regulated (reduced) to apredetermined pressure by the pressure regulating valve 520. The oilpassage LE is connected to the oil passage LM via the check valve 570.When the hydraulic pressure in the oil passage LE is higher by apredetermined pressure or more than the hydraulic pressure in the oilpassage LM, the check valve 570 opens, and the hydraulic pressure in theoil passage LE is supplied to the oil passage LM via the check valve570. Thus, during a stop of the MOP 501 as well, it is possible tosupply hydraulic pressure to the oil passage LM by driving the EOP 502.

The electromagnetic change-over valve 560 is switched to any one of anon state and an off state in response to the control signal S/C from thecontroller 100. In the on state, the electromagnetic change-over valve560 communicates the oil passage LE with the oil passage L4. In the offstate, the electromagnetic change-over valve 560 interrupts the oilpassage LE from the oil passage L4, and releases the hydraulic pressurein the oil passage L4 to the outside. FIG. 4 illustrates the case wherethe electromagnetic change-over valve 560 is in the off state.

The oil passage L4 is connected to the simultaneous supply preventionvalves 530, 540. When the electromagnetic change-over valve 560 is inthe on state, the hydraulic pressure in the oil passage LE is input tothe simultaneous supply prevention valves 530, 540 via the oil passageL4 as a signal pressure. When the signal pressure from the oil passageL4 is input to the simultaneous supply prevention valve 530, thesimultaneous supply prevention valve 530 is forcibly fixed to the normalstate irrespective of whether the signal pressure (B1 pressure) is inputfrom the oil passage L2. Similarly, when the signal pressure is inputfrom the oil passage L4 to the simultaneous supply prevention valve 540,the simultaneous supply prevention valve 540 is forcibly fixed to thenormal state irrespective of whether the signal pressure (C1 pressure)is input from the oil passage L1. Therefore, by driving the EOP 502 andswitching the electromagnetic change-over valve 560 to the on state, thesimultaneous supply prevention valves 530, 540 are simultaneously fixedto the normal state. Thus, the clutch C1 and the brake B1 are allowed tobe simultaneously engaged, and two-motor mode (described later) isenabled.

Hereinafter, the details of control modes of the hybrid vehicle 1 willbe described with reference to an operation engagement chart and thenomographs.

FIG. 5 is a chart that shows each drive mode and controlled statuses ofthe clutch C1 and brake B1 of the transmission unit 40 in each drivemode.

The controller 100 causes the hybrid vehicle 1 to travel in motor drivemode (hereinafter, referred to as EV mode), hybrid mode (hereinafter,referred to as HV mode) or engine drive mode. The EV mode is a controlmode in which the engine 10 is stopped and the hybrid vehicle 1 iscaused to travel by using the power of at least one of the first MG 20or the second MG 30. The HV mode is a control mode in which the hybridvehicle 1 is caused to travel by using the power of the engine 10 andthe power of the second MG 30. The engine drive mode is a control modein which the first MG 20 and the second MG 30 are not used and thevehicle is caused to travel by using the driving force of the engine 10.Each of the EV mode, the HV mode and the engine drive mode is furtherdivided into some control modes.

In FIG. 5, C1, B1, CS, MG1 and MG2 respectively denote the clutch C1,the brake B1, the clutch CS, the first MG 20 and the second MG 30. Thecircle mark (◯) in each of the C1, B1, CS columns indicates the engagedstate, the cross mark (×) indicates the released state, and the trianglemark (Δ) indicates that any one of the clutch C1 and the brake B1 isengaged during engine brake. The sign G in each of the MG1 column andthe MG2 column indicates that the MG1 or the MG2 is mainly operated as agenerator. The sign M in each of the MG1 column and the MG2 columnindicates that the MG1 or the MG2 is mainly operated as a motor.

In EV mode, the controller 100 selectively switches between one-motormode and two-motor mode in response to a user's required torque, and thelike. In one-motor mode, the hybrid vehicle 1 is caused to travel byusing the power of the second MG 30 alone. In two-motor mode, the hybridvehicle 1 is caused to travel by using the power of both the first MG 20and the second MG 30.

When the load of the drive system 2 is low, the one-motor mode is used.When the load of the drive system 2 becomes high, the drive mode ischanged to the two-motor mode.

As shown in E1 line of FIG. 5, when the hybrid vehicle 1 is driven(moved forward or reversed) in one-motor EV mode, the controller 100places the transmission unit 40 in the neutral state (state in which nopower is transmitted) by releasing the clutch C1 and releasing the brakeB1. At this time, the controller 100 causes the first MG 20 to mainlyoperate as fixing means for fixing the rotation speed of the sun gear S2to zero and causes the second MG 30 to mainly operate as a motor (seeFIG. 6 (described later)). In order to cause the first MG 20 to operateas the fixing means, the current of the first MG 20 may be controlled byfeeding back the rotation speed of the first MG 20 such that therotation speed becomes zero. When the rotation speed of the first MG 20is kept zero even when torque is zero, cogging torque may be utilizedwithout adding current. When the transmission unit 40 is placed in theneutral state, the engine 10 is not co-rotated during regenerativebraking, so a loss is smaller by that amount, and it is possible torecover large regenerated electric power.

As shown in the E2 line in FIG. 5, when the hybrid vehicle 1 is brakedin one-motor EV mode and engine brake is required, the controller 100engages any one of the clutch C1 and the brake B1. For example, whenbraking force is insufficient with only regenerative brake, engine brakeis used together with regenerative brake. For example, when the SOC ofthe battery 60 is close to a full charge state, regenerated electricpower cannot be charged, so it is conceivable to establish an enginebrake state.

By engaging any one of the clutch C1 and the brake B1, a so-calledengine brake state is established. In the engine brake state, therotation of the drive wheels 90 is transmitted to the engine 10, and theengine 10 is rotated. At this time, the controller 100 causes the firstMG 20 to mainly operate as a motor, and causes the second MG 30 tomainly operate as a generator.

On the other hand, as shown in the E3 line in FIG. 5, when the hybridvehicle 1 is driven (moved forward or reversed) in two-motor EV mode,the controller 100 restricts (locks) the rotation of the ring gear R1 ofthe transmission unit 40 by engaging the clutch C1 and engaging thebrake B1. Thus, the rotation of the carrier CA2 of the differential unit50 coupled to the ring gear R1 of the transmission unit 40 is alsorestricted (locked), so the carrier CA2 of the differential unit 50 iskept in a stopped state (Engine Rotation Speed Ne=0). The controller 100causes the first MG 20 and the second MG 30 to mainly operate as motors(see FIG. 7 (described later)).

In EV mode (one-motor mode or two-motor mode), the engine 10 is stopped,so the MOP 501 is also stopped. Therefore, in EV mode, the clutch C1 orthe brake B1 is engaged by using hydraulic pressure that is generated bythe EOP 502.

E4 and E5 lines in EV mode will be described. These modes as well as E3line are two-motor modes, and differ from E3 line in that these modesare operable even when the engine rotation speed Ne is not zero (Ne freein FIG. 5). The details of these modes will be described later withreference to the nomograph of FIG. 10.

The HV mode may be divided into three modes, that is, a series-parallelmode, a series mode and a parallel mode. In series-parallel mode orseries mode, the controller 100 causes the first MG 20 to operate as agenerator, and causes the second MG 30 to operate as a motor. Inparallel mode, the controller 100 causes only the second MG 30 tooperate as a motor (one-motor mode) or causes both the first MG 20 andthe second MG 30 to operate as motors (two-motor mode).

In HV mode, the controller 100 sets the control mode to any one of theseries-parallel mode, the series mode and the parallel mode.

In series-parallel mode, part of the power of the engine 10 is used inorder to drive the drive wheels 90, and the remaining part of the powerof the engine 10 is used as power for generating electric power in thefirst MG 20. The second MG 30 drives the drive wheels 90 by usingelectric power generated by the first MG 20. In series-parallel mode,the controller 100 changes the speed ratio of the transmission unit 40in response to the vehicle speed.

When the hybrid vehicle 1 is caused to move forward in an intermediateor low speed range, the controller 100 establishes the low gear positionLo (see the continuous line in FIG. 8 (described later)) by engaging theclutch C1 and releasing the brake B1 as shown in the H2 line in FIG. 5.On the other hand, when the hybrid vehicle 1 is caused to move forwardin a high speed range, the controller 100 establishes the high gearposition Hi (see the dashed line in FIG. 8 (described later)) byreleasing the clutch C1 and engaging the brake B1 as shown in the H1line in FIG. 5. Either when the high gear position is established orwhen the low gear position is established, the transmission unit 40 andthe differential unit 50 operate as a continuously variable transmissionas a whole.

When the hybrid vehicle 1 is reversed, the controller 100 engages theclutch C1 and releases the brake B1 as shown in the H3 line in FIG. 5.When there is an allowance in the SOC of the battery, the controller 100rotates the second MG 30 alone in the reverse direction; whereas, whenthere is no allowance in the SOC of the battery, the controller 100generates electric power with the use of the first MG 20 by operatingthe engine 10 and rotates the second MG 30 in the reverse direction.

In series mode, the entire power of the engine 10 is used as power forgenerating electric power with the use of the first MG 20. The second MG30 drives the drive wheels 90 by using electric power generated by thefirst MG 20. In series mode, when the hybrid vehicle 1 is moved forwardor when the hybrid vehicle 1 is reversed, the controller 100 releasesboth the clutch C1 and the brake B1 and engages the clutch CS (see FIG.9 (described later)) as shown in the H4 line and the H5 line in FIG. 5.

In HV mode, the engine 10 is operating, so the MOP 501 is alsooperating. Therefore, in HV mode, the clutch C1, the clutch CS or thebrake B1 is engaged mainly by using hydraulic pressure generated by theMOP 501.

The controlled statuses in parallel HV mode are shown in H6 to H9 lines.These are also the HV mode; however, the first MG 20 does not operate asa generator. The two-motor (parallel) HV mode significantly differs fromthe series-parallel mode or the series mode in that the first MG 20operates to carry out motoring as a motor and outputs torque forrotating the drive wheels. In parallel mode, any one of the clutch C1and the brake B1 is engaged, the other one of the clutch C1 and thebrake B1 is released, and the clutch CS is engaged. The details of thesemodes will be described later with reference to the nomograph of FIG.13.

The vehicle 1 is able to travel in engine drive mode in which thevehicle 1 travels without using the first MG 20 or the second MG 30.When the traveling state of the vehicle coincides with a rotation speedand a torque where the efficiency of the engine is high, the efficiencyis high when the power of the engine is directly used to rotate thedrive wheels without using the power of the engine for power generation,or the like. The controlled statuses in engine drive mode are shown inZ1 and Z2 lines in FIG. 5. In engine drive mode, as well as the parallelHV mode, any one of the clutch C1 and the brake B1 is engaged, the otherone of the clutch C1 and the brake B1 is released, and the clutch CS isengaged. The details of these modes will be described later withreference to the nomographs of FIG. 12 and FIG. 13.

Hereinafter, the statuses of the rotating elements in typical modesamong the operation modes shown in FIG. 5 will be described withreference to the nomographs.

FIG. 6 is a nomograph for illustrating the operation of the one-motor EVmode (E1 line in FIG. 5). FIG. 7 is a nomograph for illustrating theoperation of the two-motor EV mode (E3 line in FIG. 5). FIG. 8 is anomograph for illustrating the operation of the series-parallel HV mode(H1, H2 lines in FIG. 5). FIG. 9 is a nomograph for illustrating theoperation of the series HV mode (H4 line in FIG. 5).

In FIG. 6 to FIG. 9, S1, CA1 and R1 respectively denote the sun gear S1,the carrier CA1 and the ring gear R1 of the transmission unit 40, S2,CA2 and R2 respectively denote the sun gear S2, the carrier CA2 and thering gear R2 of the differential unit 50.

The controlled statuses in one-motor EV mode (E1 line in FIG. 5) will bedescribed with reference to FIG. 6. In one-motor EV mode, the controller100 releases the clutch C1, the brake B1 and the clutch CS of thetransmission unit 40, stops the engine 10, and causes the second MG 30to mainly operate as a motor. Therefore, in one-motor EV mode, thehybrid vehicle 1 travels by using the torque of the second MG 30(hereinafter, referred to as MG2 torque Tm2).

At this time, the controller 100 executes feedback control over thetorque of the first MG 20 (hereinafter, referred to as MG1 torque Tm1)such that the rotation speed of the sun gear S2 becomes zero. Therefore,the sun gear S2 does not rotate. However, because the clutch C1 andbrake B1 of the transmission unit 40 are released, the rotation of thecarrier CA2 of the differential unit 50 is not restricted. Therefore,the ring gear R2 and carrier CA2 of the differential unit 50 and thering gear R1 of the transmission unit 40 are rotated (coasted)interlocking with the rotation of the second MG 30 in the same directionas the second MG 30.

On the other hand, the carrier CA1 of the transmission unit 40 is keptin a stopped state because the engine 10 is stopped. The sun gear S1 ofthe transmission unit 40 is rotated (coasted) interlocking with therotation of the ring gear R1 in a direction opposite to the rotationdirection of the ring gear R1.

In order to decelerate the vehicle in one-motor EV mode, it is allowedto activate engine brake in addition to regenerative brake using thesecond MG 30. In this case (E2 line in FIG. 5); by engaging any one ofthe clutch C1 and the brake B1, the engine 10 is also rotated at thetime when the carrier CA2 is driven from the drive wheels 90 side, soengine brake is activated.

Next, the controlled status in two-motor EV mode (E3 line in FIG. 5)will be described with reference to FIG. 7. In two-motor EV mode, thecontroller 100 engages the clutch C1 and the brake B1, releases theclutch CS, and stops the engine 10. Therefore, the rotation of each ofthe sun gear S1, carrier CA1 and ring gear R1 of the transmission unit40 is restricted such that the rotation speed becomes zero.

Because the rotation of the ring gear R1 of the transmission unit 40 isrestricted, the rotation of the carrier CA2 of the differential unit 50is also restricted (locked). In this state, the controller 100 causesthe first MG 20 and the second MG 30 to mainly operate as motors.Specifically, the second MG 30 is rotated in the positive direction bysetting the MG2 torque Tm2 to a positive torque, and the first MG 20 isrotated in the negative direction by setting the MG1 torque Tm1 to anegative torque.

When the rotation of the carrier CA2 is restricted by engaging theclutch C1, the MG1 torque Tm1 is transmitted to the ring gear R2 byusing the carrier CA2 as a supporting point. The MG1 torque Tm1(hereinafter, referred to as MG1 transmission torque Tm1 c) that istransmitted to the ring gear R2 acts in the positive direction, and istransmitted to the counter shaft 70. Therefore, in two-motor EV mode,the hybrid vehicle 1 travels by using the MG1 transmission torque Tm1 cand the MG2 torque Tm2. The controller 100 adjusts the distributionratio between the MG1 torque Tm1 and the MG2 torque Tm2 such that thesum of the MG1 transmission torque Tm1 c and the MG2 torque Tm2 meetsthe user's required torque.

The controlled state in series-parallel HV mode (H1 to H3 lines in FIG.5) will be described with reference to FIG. 8. FIG. 8 illustrates thecase where the vehicle is traveling forward in the low gear position Lo(see H2 line in FIG. 5, and the continuous common line shown in thenomograph of S1, CA1 and R1 in FIG. 8) and the case where the vehicle istraveling forward in the high gear position Hi (see H1 line in FIG. 5,and the dashed common line shown in the nomograph of S1, CA1 and R1 inFIG. 8). For the sake of convenience of description, it is assumed thatthe rotation speed of the ring gear R1 is the same either when thevehicle is traveling forward in the low gear position Lo or when thevehicle is traveling forward in the high gear position Hi.

When the low gear position Lo is established in series-parallel HV mode,the controller 100 engages the clutch C1, and releases the brake B1 andthe clutch CS. Therefore, the rotating elements (the sun gear S1, thecarrier CA1 and the ring gear R1) rotate integrally with one another.Thus, the ring gear R1 of the transmission unit 40 also rotates at thesame rotation speed as the carrier CA1, and the rotation of the engine10 is transmitted from the ring gear R1 to the carrier CA2 of thedifferential unit 50 at the same rotation speed. That is, the torque ofthe engine 10 (hereinafter, referred to as engine torque Te) input tothe carrier CA1 of the transmission unit 40 is transmitted from the ringgear R1 of the transmission unit 40 to the carrier CA2 of thedifferential unit 50. When the low gear position Lo is established, thetorque that is transmitted from the ring gear R1 (hereinafter, referredto as transmission unit output torque Tr1) is equal to the engine torqueTe (Te=Tr1).

The rotation of the engine 10, transmitted to the carrier CA2 of thedifferential unit 50, is steplessly shifted by the use of the rotationspeed of the sun gear S2 (the rotation speed of the first MG 20), and istransmitted to the ring gear R2 of the differential unit 50. At thistime, the controller 100 basically causes the first MG 20 to operate asa generator to apply the MG1 torque Tm1 in the negative direction. Thus,the MG1 torque Tm1 serves as reaction force for transmitting the enginetorque Te, input to the carrier CA2, to the ring gear R2.

The engine torque Te transmitted to the ring gear R2 (hereinafter,referred to as engine transmission torque Tec) is transmitted from thecounter drive gear 51 to the counter shaft 70, and acts as driving forceof the hybrid vehicle 1.

In series-parallel HV mode, the controller 100 causes the second MG 30to mainly operate as a motor. The MG2 torque Tm2 is transmitted from thereduction gear 32 to the counter shaft 70, and acts as driving force ofthe hybrid vehicle 1. That is, in series-parallel HV mode, the hybridvehicle 1 travels by using the engine transmission torque Tec and theMG2 torque Tm2.

On the other hand, when the high gear position Hi is established inseries-parallel HV mode, the controller 100 engages the brake B1, andreleases the clutch C1 and the clutch CS. Because the brake B1 isengaged, the rotation of the sun gear S1 is restricted. Thus, therotation of the engine 10, input to the carrier CA1 of the transmissionunit 40, is increased in speed, and is transmitted from the ring gear R1of the transmission unit 40 to the carrier CA2 of the differential unit50. Therefore, when the high gear position Hi is established, thetransmission unit output torque Tr1 is smaller than the engine torque Te(Te>Tr1).

The controlled status in series HV mode (H4 line in FIG. 5) will bedescribed with reference to FIG. 9. In series HV mode, the controller100 releases the clutch C1 and the brake B1, and engages the clutch CS.Therefore, when the clutch CS is engaged, the sun gear S2 of thedifferential unit 50 rotates at the same rotation speed as the carrierCA1 of the transmission unit 40, and the rotation of the engine 10 istransmitted from the clutch CS to the first MG 20 at the same rotationspeed. Thus, electric power is allowed to be generated with the use ofthe first MG 20 by using the engine 10 as a power source.

On the other hand, because both the clutch C1 and the brake B1 arereleased, the rotation of each of the sun gear S1 and ring gear R1 ofthe transmission unit 40 and the rotation of the carrier CA2 of thedifferential unit 50 are not restricted. That is, because thetransmission unit 40 is in the neutral state and the rotation of thecarrier CA2 of the differential unit 50 is not restricted, the power ofthe first MG 20 and the power of the engine 10 are not transmitted tothe counter shaft 70. Therefore, the MG2 torque Tm2 of the second MG 30is transmitted to the counter shaft 70. Accordingly, in series HV mode,while electric power is generated with the use of the first MG 20 byusing the engine 10 as a power source, the hybrid vehicle 1 travels byusing the MG2 torque Tm2 generated by the use of part or all of thegenerated electric power.

Because the series mode is allowed to be achieved, it is possible toselect the operating point of the engine without concern for occurrenceof tooth contact noise of the gear mechanism due to engine torquefluctuations, to which attention needs to be paid in series-parallelmode, when the vehicle travels at a low vehicle speed. Thus, a vehiclestate that enables both quietness of the vehicle and improvement in fuelconsumption increases.

In series HV mode, the controller 100 sets the transmission unit 40 tothe neutral state and sets the clutch CS to the engaged state, and thencauses the first MG 20 to generate electric power by the use of thepower of the engine 10, and causes the second MG 30 to generate drivingforce for propelling the vehicle. In the above-described series HV mode,a shock at a startup of the engine 10 is interrupted by the transmissionunit 40 in the neutral state, and is not transmitted to the drive wheels90. Thus, it is possible to reduce a shock at a startup of the engine10, which is experienced by a user.

FIG. 10 is a nomograph for illustrating the operation of the two-motorEV mode (E4, E5 lines in FIG. 5). The controlled statuses in two-motorEV mode will be described with reference to FIG. 10. FIG. 10 illustratesthe case where the vehicle is traveling forward in the low gear positionLo (see the continuous common lines) and the case where the vehicle istraveling in the high gear position Hi (see the dashed common lines).For the sake of convenience of description, it is assumed that therotation speed of the ring gear R1 is the same either when the vehicleis traveling forward in the low gear position Lo or when the vehicle istraveling forward in the high gear position Hi.

When the low gear position Lo is established in two-motor EV mode (E5line in FIG. 5), the controller 100 engages the clutch C1 and the clutchCS and releases the brake B1. Therefore, the rotating elements (the sungear S1, the carrier CA1 and the ring gear R1) of the transmission unit40 rotate integrally with one another. When the clutch CS is engaged,the carrier CA1 of the transmission unit 40 and the sun gear S2 of thedifferential unit 50 rotate integrally with each other. Thus, all therotating elements of the transmission unit 40 and differential unit 50rotate integrally at the same rotation speed. Therefore, when the MG1torque Tm1 is generated in the positive rotation direction by the firstMG 20 together with the second MG 30, it is possible to cause the hybridvehicle 1 to travel by using both the motors. Because the engine 10 isnot autonomously driven in EV mode, the engine 10 is in a driven statewhere the engine 10 is driven by the torque of both the first MG 20 andthe second MG 30. Therefore, it is desirable that the open/close timingof each valve be operated such that resistance during rotation of theengine reduces.

The MG1 transmission torque Tm1 c transmitted to the ring gear R2 istransmitted from the counter drive gear 51 to the counter shaft 70, andacts as the driving force of the hybrid vehicle 1. At the same time, theMG2 torque Tm2 is transmitted from the reduction gear 32 to the countershaft 70, and acts as the driving force of the hybrid vehicle 1. Thatis, when the low gear position Lo is established in two-motor EV mode,the hybrid vehicle 1 travels by using the MG2 torque Tm2 and the MG1torque Tm1 transmitted to the ring gear R2.

On the other hand, when the high gear position Hi is established intwo-motor EV mode (E4 line in FIG. 5), the controller 100 engages thebrake B1 and the clutch CS and releases the clutch C1. Because the brakeB1 is engaged, the rotation of the sun gear S1 is restricted.

Because the clutch CS is engaged, the carrier CA1 of the transmissionunit 40 and the sun gear S2 of the differential unit 50 rotateintegrally with each other. Therefore, the rotation speed of the sungear S2 is equal to the rotation speed of the engine 10.

FIG. 11 is a nomograph for illustrating the operation of the parallel HVmode (H7, H9 lines in FIG. 5). The controlled statuses in two-motorparallel stepped HV mode will be described with reference to FIG. 11.FIG. 11 illustrates the case where the vehicle is traveling forward inthe low gear position Lo (see the continuous common lines) and the casewhere the vehicle is traveling in the high gear position Hi (see thedashed common lines).

As is apparent from the comparison between FIG. 10 and FIG. 11, intwo-motor parallel stepped HV mode, the engine 10 is autonomouslydriven, so the engine torque Te is applied to the carrier CA1 shown inFIG. 11. Therefore, the engine torque Te is also added to the ring gearR2. The remaining points of the nomograph shown in FIG. 11 are the sameas those of FIG. 10, so the description will not be repeated.

In two-motor parallel stepped HV mode, the engine torque Te, the MG1torque Tm1 and the MG2 torque Tm2 all are allowed to be used for theforward rotation torque of the drive wheels, so it is particularlyeffective when a large torque is required of the drive wheels.

The controlled statuses in one-motor parallel: stepped HV mode (H6, H8lines in FIG. 5) correspond to the case where Tm1=0 in FIG. 11. Inparallel stepped HV mode, the vehicle is allowed to travel (engine drivemode) by setting Tm1=0 and Tm2=0 and using only engine torque.

FIG. 12 is a nomograph for illustrating the operation of the enginedrive mode (Z1 in FIG. 5). FIG. 13 is a nomograph for illustrating theoperation of the engine drive mode (Z2 in FIG. 5). The nomograph of FIG.12 corresponds to a nomograph at the time when Tm1=0 and Tm2=0 in thenomograph indicated by the continuous lines in FIG. 11. The nomograph ofFIG. 13 corresponds to a nomograph at the time when Tm1=0 and Tm2=0 inthe nomograph indicated by the dashed lines in FIG. 11.

As shown in FIG. 12 and FIG. 13, the hybrid vehicle 1 further has theengine drive mode (Z1, Z2 lines in FIG. 5). In engine drive mode (Z1, Z2lines in FIG. 5), the controller 100 sets the transmission unit 40 tothe non-neutral state and sets the clutch CS to the engaged state, andthen causes the vehicle to travel by using the engine 10 in a statewhere torque is not generated by the first MG 20 or the second MG 30.

In this way, by setting the transmission unit 40 to a high-gear fixedposition or a low-gear fixed position and engaging the clutch CS, it ispossible to directly transmit the torque of the engine 10 to the driveshafts. Under the condition that the energy efficiency of the engine 10is high, fuel economy is high when the engine drive mode is used.

With the above-described control, in a state where the engine 10 isefficiently operable, the power of the engine 10 is allowed to bedirectly transmitted to the drive wheels 90 without being converted toelectric power, so it is possible to improve fuel economy.

Next, a difference in driving force among the drive modes will bedescribed. As described above, the hybrid vehicle 1 according to thepresent embodiment is able to travel in many drive modes, such as theone-motor EV mode; the two-motor EV mode, the two-motor HV mode and theengine drive mode. For this reason, it is required to study which drivemode is used in what situation.

FIG. 14 is a graph that shows the relationship between a vehicle speedand a maximum driving force in each drive mode. In FIG. 14, the line L1indicates the maximum driving force in engine drive mode, the line L2indicates the maximum driving force in one-motor EV mode, the line L3indicates the maximum driving force in two-motor EV mode, and the lineL4 indicates the maximum driving force in two-motor parallel HV mode.

The line L1 indicates the driving force at the time when the engine isset to a maximum power in the case where the vehicle travels in enginedrive mode while the engine is directly coupled to the output shaft (thegear position is Lo) as shown in FIG. 12. The line L2 indicates thedriving force generated by the torque of only the second MG 30 as shownin FIG. 6.

The line L3 indicates the driving force generated by the torque of boththe first MG 20 and the second MG 30 as shown in FIG. 7. However, whenthe vehicle speed exceeds V1, the maximum driving force reduces at astroke. This is because, when the vehicle speed becomes V1, the rotationspeed of the first MG 20 decreases in the negative direction in FIG. 7and then reaches a limit value and, as a result, the operating state ischanged. Specifically, when the vehicle speed is higher than V1, theclutch CS is engaged, and any one of the clutch C1 and the brake B1 isengaged and the other one of the clutch C1 and the brake B1 is releasedas shown in FIG. 10. Thus, the state of the first MG 20 is changed suchthat the first MG 20 generates positive torque, and the rotation speedis lower than that in the state of FIG. 7. In this state, because thetorque of the first MG 20 is not increased by the planetary gearmechanism and there is a loss for idling of the engine 10, the drivingforce remarkably reduces in a stepwise manner at the vehicle speed V1.

When a driving force larger than that in two-motor mode indicated by theline L3 is required, the torque of the engine 10 is used in addition tothe torque of both the first MG 20 and the second MG 30 as indicated bythe line L4.

In this case, the controller 100 sets the transmission unit 40 to thenon-neutral state and sets the clutch CS to the engaged state, and thencauses the vehicle to travel by using driving force from the engine 10in addition to driving force from the first MG 20 and driving force fromthe second MG 30 (H7, H9 lines in FIG. 5).

With the above-described control, it is possible to further increase themaximum driving force of the vehicle (the line L4 in FIG. 14) ascompared to the EV mode (the line L3 in FIG. 14) in which the engine 10is stopped and the first MG 20 and the second MG 30 are operated tocarry out motoring.

When the gear position is switched at the vehicle speed V1, the Lo gearindicated by the continuous line in FIG. 11 is used when the vehiclespeed is lower than V1, and the Hi gear indicated by the dashed line inFIG. 11 is used when the vehicle speed is higher than V1, with theresult that the driving force reduces in a stepwise manner.

Next, control over the clutches and the brake in the case where thedrive mode is changed to the two-motor EV mode will be described. In theabove-description, the controlled statuses of the clutches C1, CS andbrake B1 in each drive mode are mainly described. Hereinafter, controlat the time of switching the drive mode in the case where the drive modeis changed will be described.

FIG. 15 is a flowchart for illustrating control over the clutches andthe brake in two-motor EV mode, which is executed by the controller 100.As shown in FIG. 15, when the process of this flowchart is started, itis initially determined in step S10 whether the drive mode is switchedto the two-motor mode.

For example, determination as to switching of the drive mode is carriedout on the basis of a map in which ranges are determined on the basis ofa vehicle speed and a vehicle load; FIG. 16 is a view that shows anexample of such a map for determining the drive mode. As shown in FIG.16, in positive and negative low load ranges, the one-motor EV mode isused. Basically, it is not necessary to assume a startup of the engine10, a relatively wide range in which reaction force compensation torqueresulting from a startup of the engine 10 is not required may beallocated to the one-motor EV mode.

In a high load range, torque is insufficient in one-motor mode, so thetwo-motor mode is selected. That is, in a range in which the vehiclespeed is lower than a predetermined value and the load is small, theone-motor EV mode is selected; whereas, when the load is larger than apredetermined value, the two-motor EV mode is selected.

When the vehicle speed exceeds the predetermined value V1 in two-motormode, because the rotation speed of each of the first MG 20 and thepinion gears has an upper limit, the state of the vehicle changes fromthe two-motor mode (FIG. 7) in which the engine rotation speed Ne iszero to the two-motor mode (FIG. 10) in which the engine rotation speedNe is not zero.

When the vehicle speed exceeds V2, because the energy efficiency at thetime when the vehicle travels by using the electric power of the batterytends to deteriorate, any one of the series-parallel HV mode (Lo), theseries-parallel HV mode (Hi) and the series HV mode is selected.

When the drive mode is not switched to the two-motor mode in step S10,the process proceeds to step S60, and the control is returned to themain routine. On the other hand, when the drive mode is switched to thetwo-motor mode in step S10, the process proceeds to step S20.

In step S20, it is determined whether the drive mode is changed from theseries-parallel mode to the two-motor EV mode. For example, when thestate is changed from the one-motor EV mode as shown in FIG. 6 to thetwo-motor EV mode, because the engine rotation speed is zero, it isrelatively easy to directly change into the state shown in FIG. 7.However, in series-parallel mode as shown in FIG. 8, the engine rotationspeed is not zero. Therefore, in order to change from the state shown inFIG. 8 to the state shown in FIG. 7, it is required to decrease therotation speed of the engine, coasting by inertia force, to zero.Therefore, when the drive mode before a change is the series-parallelmode in step S20 (YES in S20), the process proceeds to step S50, theclutch CS is engaged, the state shown in FIG. 10 is once set, and thedrive mode is placed in two-motor EV Mode in which the engine rotationspeed Ne is not zero.

Even when the drive mode before a change is not the series-parallel modein step S20 (YES in S20), but when it is required in step S30 to set theengine rotation speed Ne to a predetermined rotation speed or higher,the process proceeds to step S50 because of a similar reason. In stepS50, the clutch CS is engaged, the state shown in FIG. 10 is once set,and the drive mode is placed in two-motor EV mode in which the enginerotation speed Ne is not zero. For example, when it is required to drivethe MOP 501 for lubrication or when it is required to avoid a rotationspeed range in which the vibration of the vehicle increases because ofresonance, it is determined that it is required to set the enginerotation speed Ne to the predetermined rotation speed or higher.

When it is not required to increase the engine rotation speed Ne in stepS30 (NO in S30), the process proceeds to step S40. In step S40, thedrive mode is switched to the two-motor EV Mode by engaging the clutchC1 and the brake B1.

When the states of the clutches C1, CS and brake B1 are determined instep S40 or step S50, the process proceeds to step S60, and the controlis returned to the main routine.

As described above, as shown in FIG. 14 to FIG. 16, the controller 100sets the drive mode to a first mode (E3 line in FIG. 5: two-motor EVmode (Ne=0)) when the vehicle speed is lower than the determinationthreshold V1, and sets the drive mode to a second mode (E5 line in FIG.5: two-motor EV mode (Ne free)) when the vehicle speed is higher thanthe determination threshold.

When the drive mode is selected as described above, even when thevehicle speed increases and the vehicle is not allowed to travel infirst mode because of the limitation of the rotation speed of the firstMG 20, the vehicle is allowed to travel with large driving force byusing the first MG 20 and the second MG 30 at the same time when thesecond mode is used.

Preferably, the hybrid vehicle 1 further has a third mode (H7, H9 linesin FIG. 5: two-motor parallel HV mode) as the drive mode. In third mode,the controller 100 sets the transmission unit 40 to the non-neutralstate and sets the clutch CS to the released state, and then causes thefirst MG 20 to generate electric power in a state where the engine 10 isoperated, and causes the second MG 30 to generate driving force forpropelling the vehicle. When the controller 100 changes the drive modefrom the third mode to the first mode, the controller 100 changes thedrive mode via the second mode.

In this way, when the drive mode is changed from the third mode to thefirst mode, it is possible not to cause a driver to experience a feelingof output torque loss by changing the drive mode via the second mode.

Subsequently, an example at the time of a change of the drive mode willbe described with reference to the operation waveform chart. FIG. 17 isan operation waveform chart that shows an example of a change from theseries-parallel HV mode to the two-motor EV mode.

As shown in FIG. 17, in the initial state at time t0, the hybrid vehicleis traveling in series-parallel HV mode. At this time, the clutch C1 iscontrolled to the engaged state, the brake B1 is controlled to thereleased state, and the transmission unit 40 establishes the Lo gearposition. The clutch CS is controlled to the released state.

From time t0 to time t1, the second MG 30 is outputting a positivetorque at a positive rotation speed, and is being operated to carry outmotoring. The first MG 20 is outputting a negative torque at a negativerotation speed, and is generating electric power through regenerativeoperation. The engine 10 is operating at a positive torque and apositive rotation speed.

At time t1, in response to the fact that the vehicle speed becomes lowerthan the threshold V2, it is determined to change the drive mode to thetwo-motor mode. At this time, the rotation speed of the first MG 20 islower than the rotation speed of the engine 10 as shown by the nomographindicated by the continuous lines in FIG. 8. When engagement of theclutch CS is started in a state where there is a difference in rotationspeed, a shock at the time of engagement is large, so the process ofsynchronizing the rotation speed of the engine 10 with the rotationspeed of the first MG 20 is executed from time t1 to time t2.

At time t2, when the rotation speed of the engine 10 and the rotationspeed of the first MG 20 are substantially equal to each other, the CSpressure begins to increase from zero. From time t2 to time t3, the CSpressure increases, the engine torque reduces, and the torque (MG1torque) of the first MG 20 changes from a negative value to a positivevalue.

At time t3, the clutch CS completes engagement, and the rotation speedsof the engine 10, first MG 20 and second MG 30 are equal to one another(the state indicated by the continuous lines in FIG. 10). After time t3,the vehicle travels in two-motor mode. At this time, the engine 10 isbeing rotated by the first MG 20 and the second MG 30, and negativetorque is indicated as rotation resistance. After time t3, when fuel isnot supplied to the engine 10 in the case where the vehicle is caused totravel in second mode (E5 line in FIG. 5: two-motor EV mode (Ne free)),the controller 100 changes the open/close timing of an intake valve orexhaust valve such that resistance during rotation of the engine 10 isreduced.

When the vehicle is caused to travel in second mode and fuel is notsupplied to the engine 10, the engine 10 is forcibly rotated by thefirst MG 20 and the second MG 30. In this case, an energy loss issmaller when the rotation resistance of the engine 10 is small. In orderto reduce the rotation resistance of the engine 10, it is desirable thatthe compressibility and expansion coefficient of air in a cylinder besmall. Therefore, the controller 100 reduces the rotation resistance ofthe engine 10 by changing the open/close timing of the intake valve orexhaust valve, thus reducing an energy loss.

Lastly, the hybrid vehicle 1 according to the present embodiment issummarized with reference to FIG. 1, and the like, again. As shown inFIG. 1, the hybrid vehicle 1 includes the engine 10, the first MG 20,the second MG 30, the transmission unit 40, the differential unit 50,the clutch CS and the controller 100. The controller 100 controls theengine 10, the first MG 20, the transmission unit 40 and the clutch CS.The controller 100 sets the transmission unit 40 to the non-neutralstate and sets the clutch CS to the engaged state, and then causes thevehicle to travel by using driving force from the first MG 20 anddriving force from the second MG 30 at the same time (FIG. 10).

By providing such a drive mode, the vehicle is allowed to be propelledby operating both the first MG 20 and the second MG 30 to carry outmotoring even in a state where the rotation speed of the engine 10 isnot zero (indicated as Ne free in E4, E5 lines in FIG. 5). Therefore, itis possible to increase the opportunity that the two rotary electricmachines are allowed to be used, so the flexibility of control over thevehicle increases in the case where large driving force is required inEV mode.

Preferably, the controller 100 switches the drive mode of the vehiclebetween the first mode (E3 line in FIG. 5) and the second mode (E4, E5lines in FIG. 5) in response to the vehicle speed. The first mode is adrive mode in which the rotation speed of the engine 10 is fixed to zeroand the clutch CS is set to the released state, and then the vehicle iscaused to travel by using driving force from the first MG 20 and drivingforce from the second MG 30 at the same time. The second mode is a drivemode in which the transmission unit 40 is set to the non-neutral stateand the clutch CS is set to the engaged state, and then the vehicle iscaused to travel by using driving force from the first MG 20 and drivingforce from the second MG 30 at the same time.

Because the second mode is provided as the drive mode as describedabove, even when the rotation speed of the engine is not zero like achange from a state where the engine is being operated to the EV mode,the vehicle is able to travel with large driving force using the firstMG 20 and the second MG 30 at the same time.

The embodiment described above is illustrative and not restrictive inall respects. The scope of the invention is defined by the appendedclaims rather than the above description. The scope of the invention isintended to encompass all modifications within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A hybrid vehicle comprising: an internalcombustion engine; a first rotary electric machine; a second rotaryelectric machine configured to output power to a drive wheel; a powertransmission unit including an input element and an output element, theinput element being configured to receive power from the internalcombustion engine, the output element being configured to output powerinput to the input element, and the power transmission unit beingconfigured to switch between a non-neutral state where power istransmitted between the input element and the output element and aneutral state where power is not transmitted between the input elementand the output element; a differential unit including a first rotatingelement, a second rotating element and a third rotating element, thefirst rotating element being connected to the first rotary electricmachine, the second rotating element being connected to the secondrotary electric machine and the drive wheel, the third rotating elementbeing connected to the output element, and the differential unit beingconfigured such that, when rotation speeds of any two of the firstrotating element, the second rotating element and the third rotatingelement are determined, a rotation speed of a remaining one of the firstrotating element, the second rotating element and the third rotatingelement is determined; a clutch configured to switch between an engagedstate where power is transmitted from the internal combustion engine tothe first rotary electric machine and a released state wheretransmission of power from the internal combustion engine to the firstrotary electric machine is interrupted, power from the internalcombustion engine being transmitted to the first rotary electric machinethough at least one of a first path or a second path, the first pathbeing a path through which power is transmitted from the internalcombustion engine to the first rotary electric machine via the powertransmission unit and the differential unit, and the second path being apath through which power is transmitted from the internal combustionengine to the first rotary electric machine, the second path beingdifferent from the first path and not including the differential unit,and the clutch being provided in the second path; and a controllerconfigured to: (i) control the internal combustion engine, the firstrotary electric machine, the power transmission unit and the clutch, and(ii) set the power transmission unit to the non-neutral state, set theclutch to the engaged state, and then cause the vehicle to travel byusing driving force from the first rotary electric machine and drivingforce from the second rotary electric machine.
 2. The hybrid vehicleaccording to claim 1, wherein the controller is configured to switch adrive mode of the vehicle between a first mode and a second mode inresponse to a vehicle speed, the first mode is a drive mode in which arotation speed of the internal combustion engine is fixed to zero, theclutch is set to the released state and then the vehicle is caused totravel by using driving force from the first rotary electric machine anddriving force from the second rotary electric machine, and the secondmode is a drive mode in which the power transmission unit is set to thenon-neutral state, the clutch is set to the engaged state and then thevehicle is caused to travel by using driving force from the first rotaryelectric machine and driving force from the second rotary electricmachine.
 3. The hybrid vehicle according to claim 2, wherein thecontroller is configured to: (i) when the vehicle speed is lower than adetermination threshold, set the drive mode to the first mode, and (ii)when the vehicle speed is higher than the determination threshold, setthe drive mode to the second mode.
 4. The hybrid vehicle according toclaim 2, wherein the controller is configured to: (i) in a third modethat is the drive mode of the vehicle, set the power transmission unitto the non-neutral state, set the clutch to the released state, and thencause the first rotary electric machine to generate electric power in astate where the internal combustion engine is operated, and cause thesecond rotary electric machine to generate driving force for propellingthe vehicle, and (ii) when the drive mode is changed from the third modeto the first mode, change the drive mode via the second mode.
 5. Thehybrid vehicle according to claim 2, wherein the controller isconfigured to, when fuel is not supplied to the internal combustionengine in the case where the vehicle is caused to travel in the secondmode, change open or close timing of at least one of an intake valve oran exhaust valve such that resistance during rotation of the internalcombustion engine is reduced.
 6. The hybrid vehicle according to claim1, wherein the controller is configured to set the power transmissionunit to the non-neutral state, set the clutch to the engaged state, andthen cause the vehicle to travel by using driving force from theinternal combustion engine in addition to driving force from the firstrotary electric machine and driving force from the second rotaryelectric machine.
 7. The hybrid vehicle according to claim 1, whereinthe controller is configured to, in a fourth mode as the drive mode ofthe vehicle, set the power transmission unit to the non-neutral state,set the clutch to the engaged state, and then cause the vehicle totravel by using driving force from the internal combustion engine in astate where the first rotary electric machine and the second rotaryelectric machine are not caused to generate torque.
 8. The hybridvehicle according to claim 1, wherein the controller is configured to,in a fifth mode as the drive mode of the vehicle, set the powertransmission unit to the neutral state, set the clutch to the engagedstate, and then cause the first rotary electric machine to generateelectric power by using power of the internal combustion engine, andcause the second rotary electric machine to generate driving force forpropelling the vehicle.
 9. The hybrid vehicle according to claim 1,wherein the power transmission unit is configured to change a ratio of arotation speed of the input element to a rotation speed of the outputelement.